Skip to main content

Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

See other formats

and polished rock surfaces, both on the nunatak and on the mountain slopes, are convincing evidence of recent great recession. The first definite evidence as to the position of the ice front is furnished by the Canadian Boundary Commission map, based upon photographs made in 1895. In June, 1899, Gilbert made photographs of the tidal end of the Nunatak Glacier, and Gannett made a map showing its position (PL LXIX, B). From Gilbert's photographs and Gannett's map, compared with the Boundary Commission map, it is evident that between 1895 and 1899 there was a recession of the glacier front of a little less than a mile. There was an even more notable retreat between 1895 and 1905.
In 1905 we were able to reoccupy the exact site of Gilbert's 1899 photograph of the tidal end of Nunatak Glacier (Sta. A, Map 4), even to the extent of locating small bowlders in the immediate foreground. This 1899 photograph is reproduced here as PL LXTV, together with our 1905, 1906 and 1909 photographs from the same site (PL LXV). The amount of thinning and recession in six years is striking, the retreat being a little over a mile. We thought then that the great, and for this region exceptional, recession of this glacier front was possibly to be attributed to a shattering of the glacier by the earthquakes of September, 1899; but in view of the fact that the recession went on at least as fast for the four years before the earthquakes, we are now inclined to believe that this was not the case.
We occupied the Gilbert site again in 1906, and at that time the glacier front just barely appeared from behind the projecting nunatak and was much thinner. It then became evident that if recession continued this site would no longer possess importance because of the recession of the glacier out of sight behind the nunatak. This we found to be the case in 1909, when we again occupied the photographic site, and, as our photograph shows (PL LXV, C), no ice appears. The total recession of the glacier between 1905 and 1909 was about five-eighths of a mile, making a recession between 1899 and 1909, of a little over a mile and a half, or at the rate of nearly 860 feet a year. That this recession is exceptionally rapid, and unexpected by Dr. Gilbert, is indicated by the fact that he chose the photographic site he did. One could scarcely have predicted such an extremely great recession of the glacier front in so short a time.
In 1905 and 1906 we photographed the Nunatak Glacier from other viewpoints so that in 1909 we had a basis for noting the recession, which could no longer be seen from the original Gilbert site. One of these photographic sites which we reoccupied was on the crest of the nunatak, looking down upon the glacier front. From here we found that the glacier had receded greatly, and the outline of its front had changed completely. By examination of the distinctive small gullies (as b, PL LXVI) cut in the gravels that rest on the steep mountain slope on the northern side of the fiord, we are able to tell exactly where the ice front stood in the two years (a, ice edge in 1905, PL LXVI, A; c, ice edge in 1909, PL LXVI, B), and it is from this site that we estimate the amount of recession since 1905, which was about half a mile. With the recession of the glaciers there has apparently been a change in the position of outflow of the subglacial streams. In 1905 and 1906 there were two such streams, the larger emerging on the north side, the smaller on the south. In 1909 the largest stream came out from beneath the glacier near the south margin, while there was a smaller one in the middle, and another small one on the north. Another notable change in 1909, as compared with previous years, was in the discharge of icebergs. Formerly icebergs were so frequently discharged that one